For user builds, use a release build id in the description if one exists

Companion change to Ic5913e587400dd38c04aa05de3a7becced77a59e

Change-Id: If5edbf56d567701f96f0ba46af9a90ffa8310c24
Merge branch 'cm-10.2' of https://github.com/CyanogenMod/android_build into cm-10.2